1. Field of the Invention
The present invention relates to a semiconductor optical member having a light transparent portion and a liquid crystal image display unit using the member, and to a method for fabricating the semiconductor optical member for use with the liquid crystal image display unit, as well as an image sensor, a view finder, an X-ray exposure mask, a pressure sensor, and other micromechanisms.
2. Related Background Art
Formation of a monocrystalline Si semiconductor layer on an insulating material is widely known as silicon on insulator ("SOI") technology, and provides many advantages which cannot be reached by bulk Si substrates for preparing conventional Si integrated circuits. More specifically, by utilizing the SOI structure, the following advantages can be obtained:
1 Dielectric isolation can be easily done to enable high degree of integration; PA1 2 Radiation hardness is excellent; PA1 3 Stray capacity is reduced to attain high speed; PA1 4 Well formation step can be omitted; PA1 5 Latch-up can be prevented; PA1 6 Fully depleted field effect transistor can be made by thin film formation. PA1 (1) After surface oxidation of an Si monocrystalline substrate, a window is formed to have the Si substrate partially exposed, and epitaxial growth is proceeded in the lateral direction with that exposed portion as the seed to form an Si monocrystalline layer on SiO.sub.2. PA1 (2) By use of the surface of an Si monocrystalline substrate itself as an active layer, SiO.sub.2 is formed therebeneath. PA1 1 an oxide film is formed on an Si monocrystalline substrate with V-grooves as anisotropically etched on the surface, a polycrystalline Si layer is deposited on the oxide film (as thick as the Si substrate), and thereafter by polishing from the back surface of the Si substrate, Si monocrystalline regions dielectrically separated by surrounding with the V-grooves on the thick polycrystalline Si layer are formed. In this method, although crystallinity is good, there are problems with respect to controllability and productivity (since it requires a process of depositing the polycrystalline Si thick as some hundred .mu.m), and a process in which the monocrystalline Si substrate is polished from the back surface to leave only the Si active layer as separated. PA1 2 An SiO.sub.2 layer is formed by ion implantation of oxygen into an Si monocrystalline substrate in a method called Separation by ion implanted oxygen ("Simox"), which is currently one of the most mature methods because of good matching with the Si process. However, since 10.sup.18 ions/cm.sup.2 or more of oxygen ions must be implanted for forming the SiO.sub.2 layer, the implantation time is very long and productivity is low. Therefore, the wafer cost is high. Further, many crystal defects remain, and from an industrial point of view, sufficient quality for preparing a device driven by minority carriers has not been attained. PA1 3 An SOI structure is formed by dielectric isolation by oxidizing porous Si. In this method, an N-type Si layer is formed on the surface of a P-type Si monocrystalline substrate in shape of islands by way of proton ion implantation (Imai et al., J. Crystal Growth, vol. 63, 547 (1983)), or by epitaxial growth and patterning; only the P-type Si substrate is made porous by anodization in the HF solution so as to surround the Si islands from the surface; and then the N-type Si islands are dielectrically isolated by accelerated oxidation. However, since the separated Si region is determined before the device steps, the degree of freedom in device design may be limited in some cases. PA1 bonding the surface side of a monocrystalline semiconductor layer formed on a porous first substrate to the insulating surface side of a light non-transparent second substrate, PA1 removing the first substrate with a treatment including wet etching, and PA1 forming a region upon which the light is incident from the second substrate side on the monocrystalline semiconductor layer by removing a part of the second substrate. PA1 a process of bonding a surface of non-porous monocrystalline layer on a first Si substrate made porous or a surface of an insulation layer formed on the non-porous monocrystalline layer onto a second Si substrate having an insulation layer on the surface thereof, PA1 a process of removing the first Si substrate made porous with a treatment including at least wet chemical etching, so as to form a monocrystalline semiconductor layer on the bonded insulation layer, PA1 a process of removing a part of the second Si substrate or a part of the insulation layer bonded with the second Si substrate with a treatment including at least wet chemical etching from the face side on which the monocrystalline semiconductor layer is not formed, until the insulating surface of the bonded insulation layer or the monocrystalline face of the monocrystalline semiconductor layer is exposed. PA1 a process of bonding a surface of anon-porous monocrystalline layer on a first Si substrate made porous or a surface of an insulating layer formed on the non-porous monocrystalline layer onto a second Si substrate having an insulating layer on the surface thereof, PA1 a process of removing the first Si substrate made porous with a treatment including at least wet chemical etching, so as to form a monocrystalline semiconductor layer on the bonded insulation layer, PA1 a process of forming an electronic device on the monocrystalline semiconductor layer, and PA1 a process of removing a part of the second Si substrate or a part of the insulation layer bonded with the second Si substrate, including at least partly a portion lying directly under the area having the electronic device formed, with a treatment including at least wet chemical etching, from the face side where the monocrystalline semiconductor layer is not formed, until the insulating surface of the bonded insulation layer or the monocrystalline face of the monocrystalline semiconductor layer is exposed.
In order to realize the many advantages in device characteristics as mentioned above, studies have been made about the method for forming the SOI structure for some 10 years. These studies are summarized in, for example, "Single-crystal silicon on non-single-crystal insulators"; edited by G. W. Gullen, Journal of Crystal Growth, volume 63, no. 3, pp. 429-590 (1983).
Also, it has been known for a long time to form the SOS (silicon on sapphire) structure by heteroepitaxy of Si on a monocrystalline sapphire substrate by CVD (chemical vapor deposition) method. This was successful to some extent as the most mature SOI technique, but has not been widely applied due to various reasons including many crystal defects because of lattice mismatching at the interface between the Si layer and the underlaid sapphire substrate, introduction of aluminum from the sapphire substrate into the Si layer, and above all the high price of the substrate and delay in enlargement of the substrate wafer size. In recent years, attempts to realize the SOI structure without use of a sapphire substrate have been made. Such attempts may be broadly classified into the two technologies shown below:
Methods for realizing the above (1) include the method in which a monocrystalline Si layer is formed directly by lateral epitaxial growth by CVD, the method in which amorphous Si is deposited and subjected to solid phase lateral epitaxial growth by heat treatment, the method in which an amorphous or polycrystalline Si layer is irradiated convergently with an energy beam such as electron beam, laser beam, etc., and a monocrystalline layer is grown on SiO.sub.2 by melting and recrystallization, and the method in which a melting region is scanned in a zone fashion by a rod-shaped heater (Zone Melting Recrystallization). Although these methods have advantages, they still have many problems with respect to controllability, productivity, uniformity and quality, and none of them has been industrially applied to date. For example, the CVD method requires the sacrificial oxidation in flat thin film formation, while crystallinity is poor with the solid phase growth method. On the other hand, with the beam annealing method, problems are involved in controllability such as treatment time by converged beam scanning, the manner of overlapping of beams, focus adjustment, etc. Among these, the Zone Melting Recrystallization method is the most mature, and a relatively larger scale integrated circuit has been made on trial, but still a large number of crystal defects such as subboundary remain, and no device driven by minority carriers has been prepared. Also, since any of those methods requires the Si substrate, excellent quality monocrystalline Si layer cannot be obtained on a transparent amorphous insulating substrate such as a glass. Concerning the method (2) using no Si substrate as the seed for epitaxial growth, the following three methods may be exemplified wherein:
As is well-known in this art, forming semiconductor elements on a light-transparent substrate is important for constituting a contact sensor serving as a light-receiving device and a projection-type liquid crystal image display. A high-quality driving device is required for further increasing the density, resolution and definition of the pixels (picture elements) of such a sensor or display. The number of switching elements for switching the pixels and the terminals in the drive circuit thereof and the peripheral circuits are so large that their interconnection is mechanically impossible. Consequently, it is desirable that semiconductor elements and peripheral driving circuits as above mentioned are created and interconnected within the same substrate through the same process, and the interconnection thereof should be made by patterning of conductive thin films so as to be connected within the normal integrated circuit, whereby through such a method the high density packaging is only made possible. Further, from the nature industrial demands of making higher quality products, it is needed to fabricate a device on the light-transparent substrate by the use of a mono-crystalline layer having excellent crystallinity.
However, on a light-transparent substrate represented by glass, an amorphous (or, at best, a polycrystalline) layer is generally formed because of the disorder of the crystal structure, and it is therefore difficult to produce a driving device having properties sufficient for the present demands or future demands because of its crystal structure having many defects. This is because the substrate has an amorphous crystal structure, and thus a monocrystalline layer of high quality cannot easily be obtained by simply depositing the Si layer. Further, any of the aforementioned methods using an Si monocrystalline substrate are unsuitable for obtaining a good monocrystalline film on a light-transparent substrate.
It should be noted that an attempt to fabricate a monocrystalline thin film Si on a light-transparent substrate by bonding a light-transparent substrate represented by a glass to an Si substrate, and polishing Si for the formation of thinner film (reported by Abe, Kuwahara, Nakazato, Uchiyama, Yoshizawa, in Electronic Information Communication Institute Technology Research Reports, SDM90-156, 77 (1990)), has not yet been successful because the difference between thermal expansion coefficients is as large as one digit.
Therefore, a liquid crystal display unit provided with an active matrix element, which has been commercialized as a flat panel display or a projection television, conventionally used the thin film transistor (TFT) in which an amorphous or polycrystalline silicon semiconductor layer was formed on the glass substrate.
FIG. 1 shows a schematic view for explaining an active matrix-type liquid crystal display unit which has been conventionally used. Reference numeral 1 is a pixel switch, 5 is a liquid crystal pixel, is a transparent substrate, 2 is a buffer portion, 3 is a horizontal shift register portion, and 4 is a vertical shift register portion. The luminance signal and the voice signal of television are compressed within a certain band width, and transmitted to the buffer portion 2 which is driven by the horizontal shift register 3 having a driving capability of its frequency. Then a signal is transferred to the liquid crystal while the pixel switch 1 is being turned on by the vertical shift register 4.
The performance required for each circuit is such that if a high-definition television is considered, a television signal is transferred to the buffer portion at a frequency of about 45 MHz, when the frame frequency is 50 Hz, the number of scanning lines is 1000, the horizontal scanning period is about 30 .mu.sec (effective scanning period 27 .mu.sec), and the number of horizontal pixels is about 1500. Also, the permissible period for the signal transfer per scanning line is 1 to 2 .mu.sec. Accordingly, the performance required for each element circuit is such that 1 the driving ability of horizontal shift register is 45 MHz or greater, 2 the driving ability of vertical shift register is 500 kHz or greater, 3 the driving ability of transfer switch which is driven by the horizontal shift register to transfer the television signal to the buffer portion is 45 MHz or greater, and 4 the driving ability of pixel switch is 500 kHz or greater. The driving ability referred to herein means that when the liquid crystal pixel is desired to have a certain number of gradations N, a voltage of not less than EQU Vm-(Vm-Vt)/N [V]
is transferred within the above period, assuming that the voltage giving the maximum or minimum transmittance of liquid crystal is Vm, and the threshold voltage of liquid crystal obtained from a V-T (voltage-transmittance) curve is Vt.
As will be clear from this, the pixel switch and the vertical shift register can be sufficient with a relatively small driving ability, while the horizontal shift register and the buffer portion require a high speed driving. Therefore, in the current liquid crystal display element, the pixel switch or vertical shift register is produced by forming a liquid crystal and a monolithic with a polycrystalline silicon or amorphous silicon TFT deposited on the glass substrate, and other peripheral circuits are produced by packaging IC chips from the outside.
In this case, to obtain a bright display image, it is necessary to increase the quantity of light transmitting through the liquid crystal pixel part. On the other hand, to package IC chips, the glass substrate must have a strength exceeding a certain value, as well as a great thickness thereof. Hence, the use of a thick glass substrate with a sufficiently higher quantity of transmitting light has led to the increase in the fabrication cost. According to the views of the present inventors, it has been found that the transparent portion of the substrate beneath the liquid crystal pixel part is necessary to be thin, and have a high light transmittance and a sufficient mechanical strength.
Additionally, it has been also found that such configuration is not limited to the liquid crystal image display unit, but is applicable to a contact-type image sensor and an X-ray mask, as well as a semiconductor optical member such as a view finder, a pressure sensor, and micro-mechanics.
On the other hand, an attempt to form peripheral circuits in monolithic with the polycrystalline silicon TFT has been made, but since the driving ability of individual TFT is small, it is necessary to increase the size of transistor or take complex measures for the circuit.
A high-quality driving device is required for further increasing the density, resolution and definition of the pixels (picture elements) of the display unit. The number of switching elements for switching the pixels and the terminals in the drive circuit thereof and the peripheral circuits are so large such that interconnection after separate fabrication of both is mechanically impossible (e.g., with the wire bonding or bump connection). Consequently, it is desirable that semiconductor elements and peripheral driving circuits as above mentioned are created within the same substrate through the same process, and the interconnection thereof should be made by patterning of conductive thin films to be connected within the normal integrated circuit, whereby through such a method the higher density packaging is only made possible. Further, from natural industrial demands toward higher quality products to be created, it is necessary to produce a device to be provided on a light-transparent substrate by using a monocrystalline layer having excellent crystallinity.
It is therefore quite difficult to produce a driving device having properties sufficient for the present demands or future demands because of its crystal structure having many defects in the amorphous Si or polycrystalline Si.
And apart from the above-described technical problems of the substrate, the present inventors have noticed that the configuration of switching element in the liquid crystal pixel part of the configuration of semiconductor element in the peripheral circuit must be designed from a new viewpoint, in order to obtain a higher performance liquid crystal image display unit.